1. Field of the Invention
The invention concerns a circuit for the amplification of electrical signals and, more particularly, an amplifier circuit, the gain of which can be controlled at will by an electrical variable (current or voltage).
2. Description of the Prior Art
There are known ways of making amplifiers the gain of which is controlled by a current or voltage. For example, in any transistor amplifier, if the load resistance of an output transistor is made to vary, a variation in gain is obtained. Consequently, if a transistor or a transistor in series with a resistor is used as a load, it is possible to set up a variable load resistance, controlled by an electrical signal driving the base of the transistor. Thus, a variable gain, controlled by the base of this load transistor, is obtained.
The variable gain can also be obtained by acting on the quiescent current of the amplifier or on the value of the fraction of the current applied to the load.
The problem with these controlled gain amplifiers is the absence of linearity between the gain control signal and the gain obtained. The gain is not proportionate to the voltage or the current which is used to make the gain vary. In certain applications, this is troublesome.
To make the gain proportionate to the signal that controls it, a correction circuit compensating for the non-linearity of the control generally has to be interposed between the gain control signal and the gain control input of the amplifier. But clearly, the total linearity obtained is a function of the quality of the compensation throughout the range of use, and it will be understood that it is difficult to compensate for the non-linearity of the amplifier with precision.
Furthermore, manufacturing variations have to be taken into account. Even if it were to be theoretically possible to match a correction circuit perfectly to the amplifier, the gain control of which has to be linearized, the batch production of the amplifier would inevitably lead to variations so much so that the correction of linearity would be imperfect in reality. In particular, in a manufacture in the form of monolithic integrated circuits, the manufacturing variations could be great.
The following is an example of a major application for which there is a felt need for an amplifier with precise and linear gain control: it is desired to make an amplifier having a gain that is directly proportionate to the absolute temperature (for example to make the input stage of a logarithmic gain amplifier). However, to obtain sufficient precision while, at the same time, having sufficient gain, it became necessary, in fact, to make a circuit using resistors with a known coefficient of temperature. Unfortunately, these resistors cannot be integrated into a monolithic circuit, and this is a major drawback.
Consequently, there is the felt need for an amplifier circuit with controlled gain that is linear as a function of the gain control signal and can be integrated into a monolithic circuit.
An object of the present invention is to propose a controlled gain amplifier circuit such as this.